et al., 2010; Kallivroussis et al., 2002; Kusek et al., 2016; Mousavi‐Avval et al., 2017; Mousavi Avval et al., 2011; Pahlavan et al., 2012; Ramedani et al., 2011; World Energy Council, 2016.
** SD stands for standard deviation.
Figure 3.3 Energy use in different steps of biodiesel production from canola and carinata as the feedstock.
Source: Cardone, M., Mazzoncini, M., Menini, S. et al. (2003).
Figure 3.4 Interconnection of water‐energy and SDGs.
3.4. THE FRAMEWORK FOR WATER‐ENERGY NEXUS IN BIO‐BASED SYSTEMS
Energy is used in water supply, and the bio‐based systems use water and energy to produce different types of biofuels and bioproducts (Figure 3.4). For developing the bio‐based systems, sustainable use of energy and water in the production of fuels and chemicals is a challenging task. Sustainability of energy and water in bio‐based systems needs to be considered collectively and cannot be effectively resolved in isolation. While addressing the SDGs, to deal with this issue, a nexus approach is needed. The water‐energy nexus is one of the most complex, yet critical, issues that the society faces. Water and energy are interconnected resources that face numerous challenges including a growing global population, economic crises, poverty, hunger, and climatic uncertainties.
Water‐energy nexus refers to the relationship of how energy is used in water supply, and how water is used in energy supply and conversion (Figure 3.5). In bio‐based systems, the main products follow the post‐harvest logistics for delivery to the conversion step, while by‐products are used for other purposes. Then, biomass is converted into biofuels and electrical energy which is used to supply water for cultivation of agricultural crops in the bio‐based systems.
Developing renewable energy sources from bio‐based systems creates additional stress on water resources. For this reason, the lack of water availability in some regions limits the development of bio‐based energy systems. Water demand for biofuels production ranges widely; for example, for the whole process of corn production and conversion to ethanol around 5 to 2138 L of water is required per L of ethanol (Mckenna, 2009); the main reason for this high variation is regional irrigation requirements. In addition to water use for crop production and conversion to biofuels, a large amount of water is needed to remove excess heat in the power plants. For example, in thermoelectric power plants, generation of 1 kWh electricity needs the evaporation of about 7.6 L of water to remove the excess heat. Thus, water is likely to become the most limiting resource in the development of bioenergy systems from bio‐based feedstocks.
Figure 3.5 Energy and water use and supply in bio‐based systems.
3.5. ASSESSING SUSTAINABILITY OF WATER AND ENERGY IN BIO‐BASED SYSTEMS
Objectively assessing the sustainability of bio‐based systems requires reliable metrics that accurately depict the particularities and complexities of these systems. Reliable assessment tools allow a fair analysis of the bio‐based processes and the comparison with other bioprocesses and non‐bioprocesses, and provide information for strategic decision‐making. Bio‐based systems are expected to be environment‐friendly. However, accurate assessments are necessary to avoid resource overexploitation or unexpected water/energy overuse due to land use change (Martin et al., 2018).
To allow comparison between different systems, standard tools and indicators are required. However, this standardization is yet to be achieved for the bio‐based systems (Bosch et al., 2015). Thus, there is a variety of tools and indicators that can be used for assessing the sustainability of bio‐based systems, including metrics designed originally for other systems, such as chemical processes. Most of the sustainability assessment tools include indicators related to energy use, but very few assess water use directly (Broeren et al., 2017). Even more infrequent are the metrics that evaluate the water–energy nexus. Indicators used to assess the sustainable use of water and energy in bio‐based systems could be classified into the sustainability triple bottom line categories from environmental, economic, and social perspectives. Combining the indicators from all three categories is important to achieve a fair assessment of the sustainability of bio‐based systems; however, most of the tools fail to include all of them (Martin et al., 2018). Most of the metrics used for bio‐based systems focus on efficient resource utilization.
Table 3.4 Selected indicators for water and energy use.*
| Indicators | Unit | Definition |
|---|---|---|
| Water use | ||
| Total water use | m3 | Total water used by a process |
| Water productivity (WP) | kg/m3 or kg/kg |
|
| Water intensity (WI) | m3/$ |
|
| Fractional water consumption (FWC) | m3/kg |
|
| Specific liquid waste | m3/kg |
|
| Energy use | ||
| Total energy use | kJ/h | Total energy consumed by a process |
| Energy efficiency (ηE) | kJ/kJ |
|
| Energy intensity (REI) | kJ/$ |
|
|
|
